uio_segflg/uio_lwp -> uio_vmspace.
- add a function to add a reference to a vmspace.
- add a macro to check if a vmspace belongs to kernel.
use VMSPACE_IS_KERNEL.
redo the previous correctly.
adapt some random parts of kernel to uio_vmspace.
- check the kcopy case in copy{in,out}_vmspace.
- simplify uiomove instead.
- adapt nfs.
- nfs_doio_read: #if 0 out "killproc if text is modified" part of
  the code as it's broken.  (a process reading the modified text is not
  necessarily a process which is using the file as a text.)
sync with head.
convert the rest of ufs.
rename VMSPACE_IS_KERNEL to VMSPACE_IS_KERNEL_P.  ("predicate")
suggested by Matt Thomas.
convert smbfs.
convert procfs.
sync with head.
adapt VND_COMPRESSION.
adapt opencrypto.
adapt dmover.
adapt wd.
adapt scsipi.
adapt cd9660.
define vmspace_kernel() rather than using proc0 directly.